Deck Contamination After an Exclusive-Use Hospital Delivery
A road vehicle operating under exclusive use has just delivered three Type A packages of technetium-99m generators to a hospital radiopharmacy. Before releasing the vehicle, the driver - who also acts as the carrier's radiation protection adviser - takes smear samples from the load bed. The average over the first 300 cm² of aluminium deck reads 8 Bq/cm² beta/gamma, and two discrete spots read 12 Bq/cm². All three packages were surveyed at the hospital dock and their surfaces read 1 Bq/cm². The vehicle is booked in 40 minutes to backload food-grade packaging for the same customer. The carrier has no wash bay, but a self-service truck wash 9 km away accepts low-level wash water under a discharge permit. The consignor's 24-hour number goes unanswered, and the dispatcher wants a decision within the hour.
1. The dispatcher asks whether the deck can simply be wiped down and the vehicle released for the food-grade backload. What is your answer, and which numbers drive it?
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No - the vehicle cannot be released on the strength of a wipe.
The governing limit for non-fixed contamination from beta/gamma emitters on a conveyance surface is 4 Bq/cm² averaged over 300 cm², applied to the whole area and to any hot spot. The results are 8 Bq/cm² average and 12 Bq/cm² at two points, so the vehicle is two to three times over the limit before any cleaning is attempted. A wipe is an action, not a release criterion; only a documented re-survey at or below 4 Bq/cm² on every sampled area clears the vehicle.
Two facts change the interpretation:
- The packages read 1 Bq/cm². The contamination probably did not come from this consignment, so it may have been carried in from an earlier load. That makes this a fleet-hygiene question, not just a one-off clean-up, and the vehicle's previous load history matters.
- The next load is food-grade packaging, an uncontrolled non-radioactive consignment. Releasing the vehicle as-is would move contamination into an uncontrolled chain.
The correct immediate step is to take the vehicle out of service, keep the load bed covered with access restricted, and plan a wet decontamination with containment of the washings - dry wiping would aerosolise and spread the contamination.
2. Before the truck wash is used, what has to be settled, and what would turn this from routine decontamination into a reportable transport incident?
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Preconditions for using the wash:
- The discharge permit must cover the specific radionuclide and the activity concentration likely to be produced. Tc-99m has a 6-hour half-life, which helps - the washings can be sampled, held for decay and re-sampled before release - but the permit, not the half-life, is what authorises the discharge.
- The washings must be contained and sampled, with a calculated activity estimate, before they go anywhere. 'It's short-lived' is not a disposal route.
- The operator needs a re-survey after wash-down and a plan for the contaminated rinsate if the permit limits are exceeded.
What makes it an incident: contamination of the conveyance is not, by itself, evidence that a package failed. A reportable transport incident arises if there is package damage, loss of contents, or package surface contamination above limits. Here the packages read 1 Bq/cm² and were intact, so the likely classification is a conveyance contamination event handled under the carrier's own procedures, with the survey results, decontamination record and waste disposition retained. The consignor and the RPO still need to be told, and the unanswered 24-hour number is itself a defect to follow up - a carrier cannot rely on a contact route that does not answer.
3. The deck is aluminium and shows pitting where the contamination sits. What does that change about the long-term decision?
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Pitting is the key detail. Contamination that has migrated into pits and oxide layers is fixed in practice but not removable by surface cleaning, so repeated smear surveys will keep drifting back above 4 Bq/cm² after each wash. Continuing to clean and re-release the same deck is a cycle that will eventually release a vehicle that is over the limit on a day nobody surveys it.
The decision therefore moves from cleaning to engineering control:
- Resurface or seal the deck, or fit a removable, impermeable liner that can be surveyed, replaced and disposed of as radioactive waste.
- Re-baseline the vehicle's survey record after treatment, and increase survey frequency for a period.
- If the deck cannot be brought reliably below 4 Bq/cm², withdraw the vehicle from Class 7 work rather than managing it by repeated exception.
This is the ALARA judgement the exam is testing: the numeric limit is satisfied only when the underlying cause is addressed, not when the last smear happens to pass.